Vacuum Ultraviolet Region Research Articles

Differentiation of structurally similar phenethylamines via gas chromatography–vacuum ultraviolet spectroscopy (GC–VUV)

The vacuum ultraviolet region includes wavelengths shorter than 200 nm. Electronic transitions of sigma and pi bonds lie in this region, which have the potential to yield structural information. Thus, a VUV detector should be able to detect nearly any molecule analyzable by gas chromatography. This study sought to determine the extent to which structurally similar phenethylamines are differentiated using their VUV spectra. Phenethylamines are a common drug class including pseudoephedrine and illicit drugs such as methamphetamine. Several phenethylamines are difficult to analyze by electron impact mass spectrometry due to their fragmentation giving the same mass to charge ratio fragments at similar ratios. While phenethylamines are generally differentiable by retention time, an extra layer of specificity is preferred in forensic analyses. A vacuum ultraviolet (VUV) spectrophotometer coupled to a gas chromatograph was used to collect VUV spectra at high frequency between 125 and 430 nm. Eight phenethylamines were analyzed for this work using GC/VUV. A calibration curve and limit of detection study was performed that indicates a limit of detection around 10 μg mL−1 and an upper limit of linearity around 1000 μg mL−1. The spectra, analyzed by Principal Component Analysis and Discriminant Analysis, indicate the ability to reliably differentiate each of the drugs from one another including structural isomers and diastereomers. Lastly, five “street” samples containing amphetamines were analyzed to demonstrate “real world” performance.

Band-to-band transitions and critical points in the near-infrared to vacuum ultraviolet dielectric functions of single crystal urania and thoria

Band-to-band transition energy parameters for single-crystal actinide samples of uranium oxide and thorium oxide were determined and compared using spectroscopic ellipsometry and critical-point dielectric function analyses. Spectroscopic ellipsometry measurements from the near-infrared to the vacuum ultraviolet spectral region were used to determine the dielectric functions of uranium oxide and thorium oxide. The critical-point structure is similar between UO2 and ThO2 but strongly blue shifted for ThO2. We find bandgap energies of 2.1 eV and 5.4 eV for UO2 and ThO2, respectively.

Editorial

Editorial

The Astrophysical Formation of Asymmetric Molecules and the Emergence of a Chiral Bias.

The biomolecular homochirality in living organisms has been investigated for decades, but its origin remains poorly understood. It has been shown that circular polarized light (CPL) and other energy sources are capable of inducing small enantiomeric excesses (ees) in some primary biomolecules, such as amino acids or sugars. Since the first findings of amino acids in carbonaceous meteorites, a scenario in which essential chiral biomolecules originate in space and are delivered by celestial bodies has arisen. Numerous studies have thus focused on their detection, identification, and enantiomeric excess calculations in extraterrestrial matrices. In this review we summarize the discoveries in amino acids, sugars, and organophosphorus compounds in meteorites, comets, and laboratory-simulated interstellar ices. Based on available analytical data, we also discuss their interactions with CPL in the ultraviolet (UV) and vacuum ultraviolet (VUV) regions, their abiotic chiral or achiral synthesis, and their enantiomeric distribution. Without doubt, further laboratory investigations and upcoming space missions are required to shed more light on our potential extraterrestrial molecular origins.

Identification of a unique VUV photoabsorption band of carbonic acid for its identification in radiation and thermally processed water-carbon dioxide ices.

Carbonic acid was synthesized within an ice containing water and carbon dioxide by irradiation of ~9 eV photons. Vacuum UltraViolet (VUV)/UltraViolet (UV) photoabsorption spectra of the irradiated ice revealed absorption features from carbon dioxide, ozone, water, carbon monoxide and oxygen in addition to a band peaking at ~200 nm which is identified to be characteristic of carbonic acid. After thermal processing of the irradiated ice leading to desorption of the lower volatile ices, a pure carbonic acid spectrum is identified starting from 170 K until sublimation above 230 K. Therefore the ~200 nm band in the VUV region corresponding to carbonic acid is proposed to be a unique identifier in mixed ices, rich in water and carbon dioxide typically encountered on planetary and satellite surfaces.

Ultrafast Molecular Spectroscopy Using a Hollow-Core Photonic Crystal Fiber Light Source.

We demonstrate, for the first time, the application of rare-gas-filled hollow-core photonic crystal fibers (HC-PCFs) as tunable ultraviolet light sources in femtosecond pump-probe spectroscopy. A critical requirement here is excellent output stability over extended periods of data acquisition, and we show this can be readily achieved. The time-resolved photoelectron imaging technique reveals nonadiabatic dynamical processes operating on three distinct time scales in the styrene molecule following excitation over the 242-258 nm region. These include ultrafast (<100 fs) internal conversion between the S2(ππ*) and S1(ππ*) electronic states and subsequent intramolecular vibrational energy redistribution within S1(ππ*). Compact, cost-effective, and highly efficient benchtop HC-PCF sources have huge potential to open up many exciting new avenues for ultrafast spectroscopy in the ultraviolet and vacuum ultraviolet spectral regions. We anticipate that our initial validation of this approach will generate important impetus in this area.

Branching Ratios of the N(2D03/2) and N(2D05/2) Spin-Orbit States Produced in the State-Selected Photodissociation of N2 Determined Using Time-Sliced Velocity-Mapped-Imaging Photoionization Mass Spectrometry (TS-VMI-PI-MS).

Branching ratios for N(2D03/2) and N(2D05/2) produced by predissociation of state selected excited nitrogen molecules in the vacuum ultraviolet region have been measured for the first time. The quantum numbers of the excited nitrogen molecule are defined by selective excitation of the nitrogen molecule in the Franck-Condon region from the ground electronic, 1Σg+, vibrational, v″, and rotational, J″ state to an excited Eu', v', J' state with a tunable vacuum ultraviolet, VUV1, laser. The neutral atoms produced by predissociation from this excited state are then selectively ionized with a second tunable VUV2 laser. Measurement of the relative populations of these two atoms formed in their spin-orbit states defines the quantum states for the atomic products. This means that the wave functions of the initial state and knowledge of the relative yields define all the experimental parameters for this series of unimolecular reactions. The ions formed by VUV2 are mass analyzed with a time-of-flight mass spectrometer and detected with a time slice velocity ion imaging mass spectrometer. In this manner, we can determine the recoil velocity associated with the predissociation process. Two different techniques are used to determine the spin-orbit ratios, namely, resonant VUV photoionization (RVUV-PI) spectroscopy and total kinetic energy release (TKER) spectroscopy determined from the image produced when the atoms are selectively ionized by VUV2 in the interaction region. The TKER spectra obtained from the lines at 110 296.25 and 110 304.96 cm-1 that couple to a newly discovered autoionization line at 129 529.4255 ± 0.0015 cm-1 prove that the lines observed in this region originate from the N(2D03/2) and N(2D05/2) atoms. Two other lines in this region at 110 286.20 and 110 299.89 cm-1 originate from the nitrogen N(4S03/2) that is photoionized in a 1+ 1 VUV-UV resonant multiphoton ionization process. The spin-orbit branching ratios have been evaluated for valence and Rydberg electronic excited states from 104 129.4 to 118 772.1 cm-1, and it shows that they are independent of the rotational and vibrational quantum numbers. They are not appreciably affected by the symmetry properties of the wave function in the Franck-Condon region of the excited states. In the energy region below 117 153.8 cm-1 the pathways at long internuclear distances appear to determine [N(2D03/2)]/[N(2D05/2)] branching ratios of ∼0.38, ∼0.62, and ∼1.04. At higher energies, TKER and RVUV-PI spectroscopy have been used to show that the average fraction of the N(2D03/2) and N(2D05/2) atoms produced in the spin-allowed channels that produce two N(2D0J) is 0.85 versus 0.15 for spin-forbidden channels. The importance and need for this information for comparison with theory and applications in astrochemistry are briefly discussed.

Tapping into Synchrotron and Benchtop Circular Dichroism Spectroscopy for Expanding Studies of Complex Polysaccharides and their Interactions in Anoxic Archaeological Wood

Circular dichroism (CD) (and synchrotron circular dichroism (SCD)) spectroscopy is a rapid, highly sensitive technique used to investigate structural conformational changes in biomolecules in response to interactions with ligands in solution and in film. It is a chiroptical method and at least one of the interacting molecules must possess optical activity (or chirality). In this review, we compare the capabilities of CD and SCD in the characterisation of celluloses and lignin polymers in archaeological wood. Cellulose produces a range of spectral characteristics dependent on environment and form; many of the reported transitions occur in the vacuum-ultraviolet region (&lt; 180 nm) most conveniently delivered using a synchrotron source. The use of induced CD in which achiral dyes are bound to celluloses to give shifted spectra in the visible region is also discussed, together with its employment to identify the handedness of the chiral twists in nanocrystalline cellulose. Lignin is one target for the design of future consolidants that interact with archaeological wood to preserve it. It is reportedly achiral, but here we review several studies in which CD spectroscopy has successfully revealed lignin interactions with chiral enzymes, highlighting the potential usefulness of the technique in future research to identify new generation consolidants.

Synchrotron-radiation vacuum-ultraviolet circular dichroism spectroscopy in structural biology: an overview.

Circular dichroism spectroscopy is widely used for analyzing the structures of chiral molecules, including biomolecules. Vacuum-ultraviolet circular dichroism (VUVCD) spectroscopy using synchrotron radiation can extend the short-wavelength limit into the vacuum-ultraviolet region (down to ~160 nm) to provide detailed and new information about the structures of biomolecules in combination with theoretical analysis and bioinformatics. The VUVCD spectra of saccharides can detect the high-energy transitions of chromophores such as hydroxy and acetal groups, disclosing the contributions of inter- or intramolecular hydrogen bonds to the equilibrium configuration of monosaccharides in aqueous solution. The roles of hydration in the fluctuation of the dihedral angles of carboxyl and amino groups of amino acids can be clarified by comparing the observed VUVCD spectra with those calculated theoretically. The VUVCD spectra of proteins markedly improves the accuracy of predicting the contents and number of segments of the secondary structures, and their amino acid sequences when combined with bioinformatics, for not only native but also nonnative and membrane-bound proteins. The VUVCD spectra of nucleic acids confirm the contributions of the base composition and sequence to the conformation in comparative analyses of synthetic poly-nucleotides composed of selected bases. This review surveys these recent applications of synchrotron-radiation VUVCD spectroscopy in structural biology, covering saccharides, amino acids, proteins, and nucleic acids.

Circular-Dichroism and Synchrotron-Radiation Circular-Dichroism Spectroscopy as Tools to Monitor Protein Structure in a Lipid Environment.

Circular-dichroism (CD) spectroscopy is a powerful tool for the secondary-structure analysis of proteins. The structural information obtained by CD does not have atomic-level resolution (unlike X-ray crystallography and NMR spectroscopy), but it has the great advantage of being applicable to both nonnative and native proteins in a wide range of solution conditions containing lipids and detergents. The development of synchrotron-radiation CD (SRCD) instruments has greatly expanded the utility of this method by extending the spectra to the vacuum-ultraviolet region below 190nm and producing information that is unobtainable by conventional CD instruments. Combining SRCD data with bioinformatics provides new insight into the conformational changes of proteins in a membrane environment.

Near-threshold excitation of the λ132.2 nm resonance line of the Tl&lt;sup&gt;+&lt;/sup&gt; ion by electron impact

Background : Among all processes of collision of electrons with atomic particles the study of the interaction between electrons and ions is extremely important and necessary for a better understanding of the basic collisional processes in plasma. In the processes of excitation, recombination, and ionization of ions by electron impact autoionization states play an important role. Their radiation decay makes an additional contribution to the photo-recombination process and leads to dielectronic recombination of an ion. Such contribution is most significant in the case of heavy many-electron ions such as the Tl + ion. Excitation of the Tl + ion by an electron impact at electron-ion collisions is practically not investigated up to date. Methods: Experiments were performed using a spectroscopic method in conditions of electron and ion beams intersecting at an angle of 90°. The ribbon electron beam (cross section 1×8 mm 2 , energy E e = 6–14 eV, current I e = (2–10)×10 –5 A) was formed by a low-energy three-electrode electron gun. The ion beam (cross section 2.5×2.5 mm 2 , energy E i = 800 eV, current I i = (5–7)×10 –7 A) was obtained using a low-voltage arc discharge source. The radiation due to decay of the ion states excited during the collisions was detected perpendicular to the plane of the beams intersection. Spectral separation of the radiation in a vacuum ultraviolet region was carried out by 70° vacuum monochromator based on the Seya-Namioka scheme. Results: Thorough measurements of the near-threshold region of the energy dependence of the effective cross section of the electron excitation of the resonance l 132.2 nm line (6 s 6 p 1 P o 1 → 6 s 2 1 S 0 ) of the Tl + ion in the 6–14 eV energy range revealed resonant features both after and before the excitation threshold of the line. The widths of the before-threshold peculiarities is defined by the dispersion of the excited electrons energy. This is indicative of their resonant nature, i.e., they are due to the formation and decay of the atoionization states (AIS) of the Tl atom. Most likely, they are related to the formation and decay of the AIS of the 6 s 6 pnp configuration ( n ≥ 7) to the excited levels of the Tl atom. The nature of the after-threshold structure indicates that it is also the result of resonance processes and is due to the formation and decay of the AIS, most likely of the 5 d 9 6 s 2 6 p 2 configuration, in the electronic channel. The absolute values of the excitation cross sections of the dielectronic satellites were determined by comparison of their intensities with the intensity of the resonance l 132.2 nm line which effective excitation cross section was obtained by normalizing the experimental data on the theoretical calculation by the tight-binding method of two states of the excitation cross section of the 6 s 6 p 1 P o 1 level of the Tl + ion at the 100 eV energy. Conclusions: The energy dependence of the effective excitation cross section of the dielectronic satellites of the Tl + ion resonance line showed that the absolute values of their excitation cross sections are ~ 10 –16 cm 2 and are of the same order of magnitude with that for the resonance line. It is shown that the main excitation mechanism of the resonance line satellites is dielectronic recombination which effectiveness essentially depends on the ratio of the probabilities of the AIS radiation and electron decay. In the case of the relativistic Tl + ion a strong configuration mixing of levels leads to significant increase of the AIS radiation decay probability.

Band gap engineering of CaxSr1-xF2 and its application as filterless vacuum ultraviolet photodetectors with controllable spectral responses

We experimentally demonstrate that the band gap of mixed crystals of calcium fluoride and strontium fluoride (CaF2 – SrF2, CaxSr1-xF2) can be engineered by modulating the composition ratio of CaF2 and SrF2. By increasing the CaF2 content, the band gap increases and the absorption edge is blue-shifted. A range of band gap energies can be obtained between 9.73 eV for pure SrF2 and 10.24 eV for pure CaF2. The first order Raman frequency shift increases linearly from 285.2 cm−1 for pure SrF2 to 327.8 cm−1 for pure CaF2. The ability to manipulate the band gap is maintained even at very low temperatures. Vacuum ultraviolet (VUV) photoconductive detectors are fabricated to explore the effect of varying composition ratios on spectral sensitivity. The spectral response of the photodetectors shifts to shorter wavelengths as the band gap increases. This allows the spectral response to be controlled by appropriately choosing the CaF2 – SrF2 ratio. Using CaxSr1-xF2 sensors also eliminates the need for extra filters to cut off unwanted longer wavelengths as the onset of their absorption occur in the VUV region. The controllable spectral response and filterless feature of CaxSr1-xF2 photodetectors provide an advantage over currently available oxide-, nitride-, and diamond-based photodetectors.

Electronic states of dibenzo-p-dioxin. A synchrotron radiation linear dichroism investigation

The UV absorbance bands of dibenzo-p-dioxin (dibenzo-1,4-dioxin, DD) are investigated by synchrotron radiation linear dichroism (SRLD) spectroscopy on molecular samples aligned in stretched polyethylene. The investigation covers the range 58,000–30,000 cm−1 (170–330 nm), thereby providing new information on the transitions of DD in the vacuum UV region. The observed polarization data enable experimental symmetry assignments of the observed transitions, leading to revision of previously published assignments by Ljubić and Sabljić (J. Phys. Chem. A 109 (2005) 8209-8217). In general, the experimental spectra are well predicted by the results of quantum chemical calculations using time-dependent density functional theory (TD–DFT). The observed absorbance in the region 58,000–55,000 cm−1 (170–180 nm) in the vacuum UV is almost entirely short-axis polarized, in pleasing agreement with the predicted spectrum.

Boris Peter Stoicheff. 1 June 1924—15 April 2010

Boris Stoicheff was a pioneer in the use of high-resolution Raman spectroscopy at the National Research Council of Canada to elucidate the structural properties of molecules. He was also one of the first scientists to apply lasers to spectroscopy, investigating spontaneous and stimulated Raman and Brillouin scattering in liquids and solids at the University of Toronto. He later extended the range of tunable coherent sources into the vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) regions down to 80 nm, allowing investigations of electronic states and their lifetimes for rare gas dimers. He authored 190 research and review articles as well as a biography of his postdoctoral mentor and later his senior colleague, Gerhard Herzberg. He used his keen insight, warm personality and strong personal skills to serve the scientific community in numerous administrative roles, including terms as president of the Canadian Association of Physicists and the Optical Society of America. His 24 PhD graduates and more than 20 postdoctoral fellows and visitors, all of whom benefitted from his strong mentoring skills and high standards, have gone on to prominent positions in academia, government and industry.

Enhancement of second harmonic generation by wavefront shaping in the scratch of BaMgF4 nanocrystal powder film

During the exploration of nonlinear crystal, such as BaMgF4 crystal for all-solid-state lasers in the vacuum ultraviolet region, many problems emerged in the scattering centers. These were caused by immature crystal-growth technology, which may bring difficulties to the periodic poling process for quasi-phase-matching. In previous studies, research has shown that nonlinear random materials can also be used for frequency conversion. In this paper, a random nonlinear process was observed when the fundamental wave is illuminated onto the scratch of the BaMgF4 nanocrystal powder film. Then, the second-harmonic waves scattered from the nonlinear turbid media are re-collected to a forward direction using feedback wavefront shaping. The method shows a repeated way to improve the conversion efficiency, which may be viable to improve the second order conversion efficiency of BaMgF4 at other wavelengths, especially in the VUV regime. Additionally, more interesting applications in random nonlinear material, such as nanocrystal ceramics, can be expected in the future.

Branching Ratio Measurements of the Predissociation of 12C16O by Time-Slice Velocity-Map Ion Imaging in the Energy Region from 106 250 to 107 800 cm-1.

Photodissociation of CO is a fundamental chemical mechanism for mass-independent oxygen isotope fractionation in the early Solar System. Branching ratios of photodissociation channels for individual bands quantitatively yield the trapping efficiencies of atomic oxygen resulting into oxides. We measured the branching ratios for the spin-forbidden and spin-allowed photodissociation channels of 12C16O in the vacuum ultraviolet (VUV) photon energy region from 106 250 to 107 800 cm-1 using the VUV laser time-slice velocity-map imaging photoion technique. The excitations to four 1Π bands and three 1Σ+ bands of 12C16O were identified and investigated. The branching ratios for the product channels C(3P) + O(3P), C(1D) + O(3P), and C(3P) + O(1D) of these predissociative states strongly depend on the electronic and vibrational states of CO being excited. By plotting the branching ratio of the spin-forbidden dissociation channels versus the excitation energy from 102 500 to 110 500 cm-1 that has been measured so far, the global pattern of the 1Π-3Π interaction that plays a key role in the predissociation of CO is revealed and discussed.

Electronic and Optical Properties of Two-Dimensional α-PbO from First Principles

We performed first-principles calculations based on density functional theory and many-body perturbation theory to investigate the electronic and optical properties of monolayer, bilayer, and bulk litharge α-PbO, including spin–orbit coupling effects. The fundamental gap is direct for the monolayer (4.48 eV) and indirect for the bilayer (3.44 eV) and the bulk material (2.45 eV). The exciton binding energies are large for the monolayer (1.1 eV) and the bilayer (0.9 eV), indicating that excitons are stable at room temperature. However, the lowest-energy excitons for the monolayer and the bilayer are dark with radiative lifetimes on the order of milliseconds. A pronounced van Hove singularity in the valence band of the few-layer structures suggests it becomes a multiferroic two-dimensional material upon hole doping. Our results indicate strong optical absorbance in the vacuum UV region and transparency in the visible and near UV for monolayer PbO, suggesting applications for atomically thin solar-blind UV ph...

Detection of Vacuum Ultra-Violet light by means of SiPMs

Abstract Silicon Photo-Multipliers (SiPMs) are widely proposed as light detectors in many high energy physics experiments. The performance of these devices for the detection of scintillation light of liquefied noble gases , such as LXe or LAr, emitting photons in the Vacuum Ultra-Violet (VUV) region, will be presented.

Tunable VUV photochemistry using vacuum ultraviolet free electron laser combined with H-atom Rydberg tagging time-of-flight spectroscopy.

In this article, we describe an experimental setup for studying tunable vacuum ultraviolet photochemistry using the H-atom Rydberg tagging time-of-flight technique. In this apparatus, two vacuum ultraviolet laser beams were used: one is generated by using a nonlinear four-wave mixing scheme in a Kr gas cell and fixed at 121.6 nm wavelength to probe the H-atom product through the Lyman α transition and the other beam, produced by a seeded free electron laser facility, can be continuously tunable for photodissociating molecules in the wavelength range of 50-150 nm with extremely high brightness. Preliminary results on the H2O photodissociation in the 4d (000) Rydberg state are reported here. These results suggest that the experimental setup is a powerful tool for investigating photodissociation dynamics in the vacuum ultraviolet region for molecules involving H-atom elimination processes.

Electronic Stark effect for a single molecule: Theoretical UV response

Abstract Theoretical analysis of optical absorption in the vacuum ultra-violet (VUV) region by single molecules placed in external electrostatic fields reveals characteristic absorption spectra completely at variance with their ‘zero-field’ counterparts consequent to appearance, disappearance, splitting, merging and shifts of absorption bands and extensive intensity redistributions. This ‘molecular electronic Stark effect’ can be tracked through electronic excitations involving frontier orbitals, energy-level splitting and shifts, molecular geometry changes and re-orientation, orbital mutation and migration. The UV spectra predicted herein should provide theoretical grounds for experimental endeavors on appraisal and characterization of the effect.